The ultrastructure of isolated cell walls of Saccharomyces cerevisiae from the log and stationary phases of growth was studied after treatment with the following enzymes: purified endo-ß-(1 → 3)-glucanase and endo-ß-(1 → 6)-glucanase produced by Bacillus circulans; purified exo-ß-glucanase and endo-ß-(1 → 3)-glucanase produced by Schizosaccharomyces versatilis; commercial Pronase. While exo-ß-glucanase from S. versatilis had no electron microscopically detectable effect on the walls, Pronase removed part of the external amorphous wall material disclosing an amorphous wall layer in which fibrils were indistinctly visible. Amorphous wall material was completely removed by the effect of either endo-ß-(1 → 3)- or endo-ß-(1 → 6)-glucanase of B. circulans or by a mixture of the two enzymes. As a result of these treatments a continuous fibrillar component appeared, composed of densely interwoven microfibrils resisting further action by both of the B. circulans enzymes. The fibrillar wall component was also demonstrated in untreated cell walls by electron microscopy after negative staining. Because of the complete disappearance of the fibrils following treatment with the S. versatilis endo-ß-(1 → 3)-glucanase it can be concluded that this fibrillar component is composed of ß-(1 → 3)-linked glucan. Bud scars were the only wall structures resistant to the effect of the latter enzyme.

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